1. Field of the Invention
The present invention relates to a printer incorporating a device for zeroing a carriage which has a print head mounted thereon.
2. Discussion of the Prior Art
There is known a printer of the type indicated above, wherein the home position or zero point of the carriage or a stepper motor to drive the carriage is detected or established upon power application to the printer, by using a zero-point sensor disposed within a range of movement of the carriage. In this first type of carriage zeroing arrangement, the zero-point sensor should be positioned with extremely high accuracy. A second known type of carriage zeroing arrangement uses a carriage stop disposed on a side frame of the printer, so that the carriage is zeroed by energizing the carriage drive stepper motor to move the carriage into abutting contact with the carriage stop and further energizing the motor in a step-out or out-of-synchronization manner, i.e., without an angular or linear displacement thereof with the carriage held in abutting contact with the stop. In this case, the stepper motor should be energized by a relatively large number of steps for zeroing the carriage or stepper motor. Consequently, the carriage zeroing operation produces a large amount of noise. In view of the above drawbacks of the carriage zeroing arrangements, there is known a third type of zeroing arrangement which employs a position sensor for detecting a detection point of the carriage near the carriage stop. In this case the carriage drive stepper motor is energized by a suitable number of steps to move the carriage toward the carriage stop after the detection point of the carriage is detected by the position sensor, so that the carriage drive stepper motor is energized without an angular or linear displacement thereof with the carriage held in abutting contact with the stop. This third type has a relatively large tolerance of positioning error of the position sensor, or permits relatively easy positioning of the position sensor, as compared with the first type wherein the carriage is zeroed only by means of the zero-point sensor indicated above. Further, the third type is advantageous for a relatively short time of out-of-synchronization energization of the stepper motor, as compared with the second type of zeroing arrangement which does not use the position sensor disposed near the carriage stop.
In the third type of carriage zeroing arrangement, the number of energizations of the stepper motor after the carriage detecting point is detected by the position sensor should be a sum of the number of steps which corresponds to a maximum distance between the carriage stop and the position of the position sensor which varies within a given range of positioning tolerance, and a suitable number of additional steps necessary for the out-of-synchronization energization of the stepper motor without its displacement with the carriage held in abutting contact with the carriage stop. If the position sensor is positioned relatively near the carriage stop, the stepper motor is energized by an unnecessarily large amount, and the time of the energization of the motor in the out-of-synchronization manner is unnecessarily long. Accordingly, the printer using the third type of carriage zeroing arrangement suffers from a large amount of carriage zeroing noise due to the energization of the stepper motor with the carriage in abutting contact with the carriage stop.
While the carriage or stepper motor should be zeroed upon application of power to the printer, the carriage zeroing operation is necessary in other situations. For instance, the carriage should be zeroed when a ribbon cassette on the carriage is changed with the top cover of the printer removed, because the previously established zero point of the carriage might be lost. In the light of this situation, the printer is generally adapted to re-establish the zero point of the carriage when the top cover is opened and closed. According to the third type of carriage zeroing arrangement, the carriage zeroing operation upon the closure of the top cover also requires a long period of energization of the stepper motor and suffers from an accordingly large amount of zeroing noise.